The present invention relates to the field of drinking water treatment and, more particularly, to the field of hydrogen sulfide scrubbing in a drinking water treatment system.
Drinking water is typically extracted from an aquifer and processed in a water treatment system to remove undesired contaminants and impurities, and add a sanitizer, such as chlorine, for example. Water taken from an aquifer may contain a high hydrogen sulfide content, because of surrounding geological features and/or the action of certain bacteria. Unfortunately, hydrogen sulfide gas has an unpleasant odor and it is undesirable to discharge into the atmosphere that can then annoy residents of surrounding neighborhoods, for example.
Some water treatment plants remove or scrub hydrogen sulfide from the water with a caustic scrubbing solution, such as including potassium hydroxide or sodium hydroxide. In such a system, water pumped from the aquifer is first passed through an aerator for extracting a hydrogen sulfide-containing gas flow from the water. The hydrogen sulfide-containing gas flow is then passed through a hydrogen sulfide scrubber including at least one scrubber tank through which the caustic scrubbing solution is circulated and constantly replenished.
A common configuration of a hydrogen sulfide scrubber includes two scrubber tanks each having generally lightweight scrubber media therein. The scrubber tanks may be as offered by the Duall Division of Met-Pro Corporation of Owosso, Mich. under the model series designation PT-500. The scrubber media may be in the form of hollow spheres with passageways therein to provide a large surface area to enhance the capture of hydrogen sulfide from the gas flow and into the scrubbing solution. The hydrogen sulfide scrubber may also include first and second scrubbing solution circulators, each dispensing a scrubbing solution into contact with the scrubber media, a sump for collecting the scrubbing solution, and a circulating pump for circulating the scrubbing solution from the sump and back to the dispenser. The scrubbing solution includes caustic to adjust the pH to a desired level at which the hydrogen sulfide will more readily dissolve into the scrubbing solution.
The caustic material, however, causes a build-up in the scrubber tank, and especially on the scrubber media. This build-up covers and may block the passageways in the scrubber media and reduce the available surface area for extracting the hydrogen sulfide. The build-up also increases the weight of the scrubber media and reduces movement during scrubbing. Moreover, the increased weight may also stress the supporting structure of the tank.
Periodic cleaning is recommended for caustic-based hydrogen sulfide scrubbers. Such cleaning is generally performed by acid washing and is relatively difficult. Accordingly, maintenance may be postponed until the removal of hydrogen sulfide is no longer acceptable. At this point, very costly replacement of the scrubber media may be needed. The use of caustic further requires careful handling, and is relatively expensive when the cost of removing the build-up is considered. The use of caustic also requires a considerable flow of make-up water and a corresponding relatively large discharge of spent scrubber solution into the sewer system.
In view of the foregoing background, it is therefore an object of the present invention to provide a cost effective and relatively simple process to treat hydrogen sulfide in a drinking water system while reducing build-up and the need for maintenance.
This and other objects, features and advantageous of the present invention are provided by a drinking water treatment system which uses triazine compound in the scrubber solution. For example, the triazine compound may be SCRUB-IT(trademark). More particularly, the system may comprise a pump for pumping water from an aquifer, an aerator connected downstream from the pump for generating an outlet water flow and a hydrogen sulfide-containing gas flow by extracting hydrogen sulfide from the water, and a sanitizer for sanitizing the outlet water flow from the aerator to make drinking water.
The drinking water system may further comprise a hydrogen sulfide scrubber for scrubbing the hydrogen sulfide-containing gas flow from the aerator. The hydrogen sulfide scrubber may comprise at least one scrubber tank and scrubber media therein. The scrubber may also include a scrubbing solution circulator comprising a dispenser for dispensing scrubbing solution into contact with the scrubber media, a sump for collecting the scrubbing solution after contact with the scrubber media, and a circulating pump for circulating the scrubbing solution from the sump back to the dispenser after contact with the scrubber media.
Moreover, the hydrogen sulfide scrubber may further comprise a triazine compound supply for supplying the triazine compound to the scrubbing solution. The triazine compound effectively reduces the hydrogen sulfide content without causing extensive build-up within the tank and on the media as does caustic, for example.
The at least one scrubber tank may include first and second scrubber tanks. The first scrubber tank may have a gas flow inlet connected to the aerator to receive the hydrogen sulfide-containing gas flow therefrom, and a gas flow outlet. The second scrubber tank may have a gas flow inlet connected to the gas flow outlet of the first scrubber tank. In other words, a two-stage scrubber may be used. Accordingly, a first scrubbing solution circulator may use a first scrubbing solution in the first scrubber tank, and a second scrubbing solution circulator may use a second scrubbing solution in the second scrubber tank.
In accordance with another important aspect of the invention, the triazine compound supply may be connected to only the second scrubbing solution circulator. A scrubbing solution charging line may be included for supplying a portion of the second scrubbing solution to the first scrubbing solution to charge the first scrubbing solution with the triazine compound. This arrangement provides efficient scrubbing yet reduces consumption of the triazine compound.
The drinking water treatment system may still further comprise a controller for controlling the triazine compound supply and hydrogen sulfide sensors associated with the first and second scrubber tanks that are connected to the controller. The controller controls the triazine compound supply based upon the hydrogen sulfide sensors.
Yet another aspect of the invention relates to the sensing of hydrogen sulfide. In particular, the hydrogen sulfide sensors may comprise an inlet gas sensor which includes a sampling tube having an inlet connected in fluid communication with the hydrogen sulfide-containing gas flow from the aerator to the first scrubber tank. The hydrogen sulfide sensor may also include a hydrogen sulfide sensing device positioned remote from the aerator and connected to the outlet of the sampling tube. A purge pump may be connected to the sampling tube adjacent the outlet thereof. A condensation drain valve may also be connected to the sampling tube to drain accumulated condensation from within the sampling tube. The controller may periodically operate the purge pump and the condensation drain valve to enhance the accuracy of the readings. The sensor, purge pump, and drain valve may be positioned within a housing of the controller. A similar outlet gas sensor may also be provided which includes a sampling tube having an inlet connected in fluid communication with a discharge gas flow from the second scrubber tank.
The drinking water treatment system may further comprise a make-up water supply connected to the second scrubbing solution circulator. In some embodiments, the hydrogen sulfide scrubber may further comprise a caustic supply and a switchover valve arrangement for permitting selective alternate operation using the caustic supply or the triazine compound supply. This permits a scrubber to be operated using either scrubbing solution chemistry.
One method aspect is for operating a hydrogen sulfide scrubber using a triazine compound in the scrubbing solution. Another method aspect of the present invention is for retrofitting a hydrogen sulfide scrubber of a drinking water treatment system from a caustic supply to a triazine compound supply. The method may comprise disconnecting the caustic supply for the first and second scrubber tanks, connecting the triazine compound supply for the second scrubber tank, and connecting a charging line between the second scrubber tank and the first scrubber tank.